In the product characteristics measurement of TFT-LCDs (Thin Film Transistor Liquid Crystal Display), the optical gray scale measurement devices are often used. In the product characteristics measurement, the luminance of the lamp is required to be stable, such that the gray scale of patterns outputted by TFT-LCDs are stable. By this way, patterns can be matched accurately and the characteristics values measured are stable. However, in actual product characteristics measurement, patterns matching failure and characteristics values instabilities often occur as a result of the attenuation of the lamps.